My huskee riding mower 17.5 hp briggs ! Starts missing running bad when I engage the mower deck

Hi,
The best way to approach this is to acknowledge what happens when the deck is engaged. The engine has to work harder because of the extra load which requires more fuel, a strong ignition and good air flow. This means that anything disrupting one or more of these will result in poor performance. First thing to try is a new plug gapped at 0.030"
Peace,
Clarke